Marine oil spills, the release of oil into ocean or coastal waters, often occur when oil tankers, underwater pipelines, offshore oil rigs, or offshore oil wells are damaged, resulting in a breach of oil containment. As the oil is released from its source, it forms a film, or oil slick, on the surface of the water that expands horizontally. The rate of expansion of the oil slick is governed by factors such as the viscosity, surface tension and specific gravity of the oil.
Marine oil spills are a major environmental concern. Oil spills both physically smother marine life with a coating that is difficult to remove and expose it to toxic substances contained within the oil. These toxic substances then enter the food chain and become harmful to marine life not directly contacted by the oil spill, having far reaching implications for the entire marine ecosystem. Oil spills further impact the marine ecosystem by blocking the entry of sunlight into marine waters, reducing photosynthesis of marine plants and phytoplankton. Over time, this also can have far reaching effects on the marine food chain.
One example of the dramatically harmful effect of oil spills on marine organisms is the effect of oil spills on marine birds. When birds are physically exposed to the oil, their feathers become matted, leaving them unable to fly and susceptible to drowning due to decreased buoyancy. They also have reduced insulation capacity. When the birds preen, they ingest the toxins present in the oil. The toxins can cause damage to multiple organ systems and ingestion typically results in death. Even if the marine birds do not directly come into contact with the oil spill, local prey fish populations may become contaminated with the toxins in the oil, again causing the birds to ingest the toxins and become ill.
Because of the harmful and far-reaching effects of oil spills on the marine environment, it is critical to clean spills as thoroughly and as quickly as possible. Current methods for cleaning oil spills include traditional methods of mechanically containing and removing the spill and more recent chemical and biological approaches.
Traditional methods for cleaning oil spills require first physically containing the spill. This is usually done by surrounding the spill with a series of booms. Booms are flotation devices that include both above and below-water projections to contain the oil. Once the contaminated water is contained, it is removed using one of several methods.
One option is to use a skimmer, a device for mechanically recovering the oil layer from the water surface. A second option is the use of sorbents. Sorbents are dry materials that soak up the oil, resulting in a semi-solid mixture that facilitates oil collection. Yet another option is controlled burning of the oil layer off of the water surface.
The main drawback of mechanical methods for cleaning oil spills is that they are most effective in calm waters. When seas are rough, waves carry oil over the booms, allowing it to escape into the surrounding environment. Skimmers are no longer effective in rough waters. Controlled burning must also be done in calm seas.
More recent innovations in cleaning oil spills include biological and chemical approaches. The main biological approach is bioremediation. If an oil spill is left untreated, biodegradation will eventually occur, typically in a process by which microorganisms break down the oil as they consume it for nutrients. Bioremediation is a series of techniques for speeding up this process, either by adding nutrients to the environment to speed up the oil-degrading activity of existing microorganisms, or by adding additional oil-degrading microorganisms to the environment. While bioremediation is a valuable technique, it is a long-term process that is often employed after other methods of oil spill clean-up have been exhausted, rather than a replacement for other methods.
The use of dispersing agents is a chemical approach for breaking up oil spills. Generally, dispersants are chemical compounds containing surfactants. They function by stabilizing oil droplets that form when the seawater and oil are mixed rapidly. These droplets remain dispersed in the water column, where they are then degraded by natural processes such as bioremediation. Although the oil is not physically removed from the marine environment, smaller oil droplets scattered by currents cause far less harm and degrade more easily than an oil slick. When spills occur below the sea surface, subsurface dispersants can be applied before the oil reaches the surface. Although an effective method for dispersing an oil spill, there are several concerns with the current use of dispersants. One concern relates to the (premature) release of the surfactant from the oil, before degradation has occurred. Another concern relates to the toxicity of many existing dispersants, further endangering the marine environment. Also, the dispersant is diluted into such a large volume it can be difficult to achieve a threshold concentration necessary for the droplets to be stabilized; even if formed, the emulsion will tend to destabilize due to low background concentrations caused by dilution. And in many cases, surfactants used are not ‘active’ for polycyclic aromatic hydrocarbons.